1. Field of the Invention
The present invention relates to a radio receiver and particularly to an automatic gain control ( agc ) system.
For the sake of convenience the term "radio receiver" is also intended to cover the receiver sections of transceivers.
2. Description of the Related Art
In order to operate in the present day increasingly crowded radio frequency environment in which wanted and unwanted adjacent radio channels frequently have strong signals, it has become a necessary for radio receivers, especially those with low or zero intermediate frequencies, to have agc systems which attenuate the received signals and/or vary the gain of an input rf amplifier. Analog agc systems are known in which corrections are done continuously but in order to provide stability and suppress interference such systems have a relatively long time constant which is a disadvantage for receivers operating on a time division basis where for reasons of battery conservation the receiver is switched-off for those time periods when it is known that a signal will not be sent to that receiver. An example of such a receiver is a paging receiver operating in accordance with the CCIR Radiopaging Code No.1 (otherwise known as POCSAG) in which once the receiver has achieved bit synchronization, it is powered up for one code word period in every batch of 17 code words in order to detect the sync. code word after which it is powered down. The remainder of the batch comprises 8 frames, each frame comprising 2 code words. A pager is allocated to a predetermined frame in a batch. Just prior to the occurrence of its frame the receiver is powered up in order to be able to receive any address messages transmitted in that frame. Following expiration of the predetermined frame, the receiver is powered down until just prior to the occurrence of a sync. code word in the next following batch. The cycle of powering -up and -down continues until transmissions have ceased.
An advantage of using a zero IF architecture for making radio receivers, especially for use in compact equipment such as pagers, is that they can be fabricated as integrated circuits. However one of the biggest problems for applying agc to zero IF receivers is that of self reception, that is the reception of the receiver's own local oscillator (LO) due to leakage or of any unmodulated on-channel signal, which produces an unwanted dc offset voltage at the outputs of the mixers. Variations in the strength of the input signal leads to changes in gain of the input amplifier which causes the dc offset voltage to change which leads to the generation of a transient. The transient is detected by a level detector of the agc circuit as a rising signal which can cause instability. Changes in the gain when a weaker signal is being received leads to an amplification of the self received LO signal which causes a change in the unwanted dc offset voltage. Merely changing the orientation of the receiver's antenna relative to its local environment can lead to instability.
FIGS. 1 and 2 of the accompanying drawings illustrate respectively a radio receiver having a single switched agc loop and the associated transfer function. For convenience of illustration, the radio receiver is a superheat receiver which comprises an antenna 10 which is coupled to a variable gain rf amplifier 12 which has an agc signal input 14. An output of the rf amplifier 12 is connected to a first input of a mixer 16. A local oscillator 18 is coupled to a second input of the mixer 16. A post-mixer amplifier 20 is coupled to an output of the mixer 16. A filter 22, for example a low pass filter, selects the desired band of signals from the products of the mixing operation and supplies them to an output terminal 24. An agc circuit has an input coupled to the output of the filter 22, which input is coupled to a level detector circuit 26 which produces a voltage indicative of the level of the voltage at the output of the filter 22. A delay stage 28 having a time constant r is coupled between an output of the level detector 26 and a non-inverting input of a comparator 30.
A reference voltage V.sub.ref is applied to the inverting input of the comparator 30.
An output of the comparator 30 is applied to the gain control input 14 of the rf amplifier 12. The comparator 30 behaves as a switch with hysteresis.
Referring now to FIG. 2, if the signal from the delay stage is increasing from a low value which is outside the control range, the agc circuit behaves with maximum input gain. However, when the input voltage reaches a predetermined level Vx1, corresponding to an output voltage of V.sub.out1, a fixed level of attenuation is switched into the circuit causing the output voltage to drop to a value Vy1. If the input voltage continues to increase, the output continues to increase.
Conversely when the input voltage decreases to another predetermined level Vx2, the fixed level of attenuation is switched out of the circuit causing the output voltage to rise to a value of Vy2.
In operation it is only during the switching process that the loop is closed, otherwise the loop is effectively open. As a result the time constant T of the delay circuit 28 can be much smaller and also an improved stability under self-reception is improved significantly. In spite of these improvements, a single switched agc circuit has only a limited range of controlling the input voltage level, namely between V.sub.in2 and V.sub.in1 which in turn is dependent on the limits of the output namely V.sub.out2 to V.sub.out1, where V.sub.out2 is limited by receiver sensitivity considerations and V.sub.out1 is determined by maximum permitted input level having regard second and third order intermodulation products IP2 and IP3.